There are many types of packages used to manage the thermal, optical and electrical connections to small optoelectronic devices. This is due to the inherent conflict between thermal conductivity and electrical insulation when selecting suitable materials for packaging. Typical Optoelectronic devices include lasers, LEDs, detectors and photovoltaic devices. The largest challenges associated with the packaging come from electrically pumped light emitting devices. The associated wall plug efficiency of the device dictates that the conversion from electrical energy to photons out is never unity thereby introducing substantial heating in the vicinity of the small format device.
Of particular interest to the present invention are semiconductor light emitting diodes (LED) and lasers. These comprise of a chip level small format device and are typically driven at high drive currents resulting in high power densities generated inside the material causing local heating effects.
Light emitting diodes are based on a forward biased p-n junction. LEDs have recently reached high brightness levels that have allowed them to enter into new solid state lighting applications as well as replacements for high brightness light sources such as light engines for projectors and automotive car headlights. These markets have also been enabled by the economical gains achieved through the high efficiencies of LEDs, as well as reliability, long lifetime and environmental benefits. These gains have been partly achieved by use of LEDs that are capable of being driven at high currents and hence produce high luminous outputs while still maintaining high wall plug efficiencies.
The efficiency of the LED is critical to ensure that solid state lighting is adopted for general lighting applications and be able to fulfil the environmentally friendly lighting solution for future generations. LED lighting has the potential to be up to 20 times more efficient than the incandescent light bulb and last 50-100 times longer (lasting up to 100,000 hrs) resulting in less physical waste, large energy savings and lower cost of ownership. Solid state lighting applications require that LEDs exceed efficiencies currently achievable by alternative fluorescent lighting technologies.
The current state-of-the-art chip performance is by Nichia, who quote a figure of 150 lumen per Watt of electrical drive (LPW) for a 0.1 W chip. Semileds quote a 100 LPW for a 1 W chip. Phillips quote a figure of 115 LPW for a 1 W chip. The theoretical maximum is between 260 LPW and 330 LPW depending on the colour temperature of the white light generated. The present LPW efficacy at 1 W drive amounts to a wall plug efficiency of 38% thus >60% of the electrical drive current is converted to heat. Typically so called power chips are about 1 mm sq and are driven between 1-3 W. This amounts to a thermal load density of 0.6 to higher than 1.8 W/mm2. This is a high figure compared to any other semiconductor device and leads to the need to provide specific high performance packaging solutions. To date most packaging has been adapted from the IC industry where thermal densities are orders of magnitude lower 1-3 W/cm2.
It is also of particular interest to maintain the small format light emitting device at a low temperature during operation as the junction temperature of the LED dramatically affects both its life time and its overall efficiency. As a basic rule every 10° C. increase (above 25° C.) in junction temperature reduces the life time of the LED by 10 kHrs for a Galium Nitride LED. It is also a consequence of the increase of the junction temperature that the overall efficiency of a state of the art vertical type LED drops, for example, increasing the junction temperature from 40° C. to a 70° C. will reduce the efficacy of the LEDs by more than 10%. It is noted that this effect increasingly becomes nonlinear in behaviour. Thus, appropriate packaging solutions need to be developed to ensure performance is maintained and the operating temperature of the light emitting device is maintained for a given change in the junction temperature as well as the ambient temperature.
The Thermal Resistance of a package is the measure of how well a package can conduct heat away from the junction of the LED. Current state of the art modules have a thermal resistance of between 4 and 8 K/W.
Many methods have been successfully employed to improve the thermal resistance of LED module packages. These include the use of shaped metal lead frames in array formats U.S. Pat. No. 6,770,498, the use of bulk Aluminium Nitride ceramic tiles with electrical tracking on top in U.S Patent Application 2006/0091415A1 and the use of flip chip LEDs onto tracked ceramic tiles with through vias to allow surface mounting U.S Patent Application 2006/0091409A1.
The LEDs themselves have been engineered to produce a low thermal resistance path from the junction to the package where the heat is spread such as the flip chip approach described above (U.S Patent Application 2006/0091409A1) where the junction is very close to the package. Another approach to provide LEDs with high current and thermal driving capabilities the vertical type n-p contact configuration in GaN material systems has been recently adopted an example of which has been disclosed in U.S. Pat. No. 6,884,646 and published U.S. Patent application 20060154389A1. The disclosed devices use high thermal conductivity materials such as Copper to provide low thermal resistance from the junction to the package. More recently, improvements to these vertical type LED designs with respect to optical extraction performance promise even greater wall plug efficiency chips, as described in UK patent applications 0704120.5 and 0714139.3.
Insulated Metal Core Printed Circuit Boards
The use of insulated metal substrate printed circuit boards (IMS-PCB) are common place and are as described in U.S. Pat. No. 4,810,563. These are use in many applications including LEDs. The structure of this approach is shown in FIG. 1.
The metal substrate 100 is commonly Aluminium or copper and ranges in thickness between 0.5 mm to 3.2 mm. On top of the substrate is an adhesive layer 101 typically consisting of particulate loaded epoxy. The particulates are chosen to increase the thermal conductivity and include Aluminium Nitride, Diamond and Beryllium Nitride. The choice of materials for the adhesive layer 101 is important as the IMS PCB will undergo solder operations with temperatures used during reflow being as high at 320° C.
Layer 102 is a polyamide film. On top of this is an electrical circuit layer 103 that usually consists of copper. Layer 103 has two functions, one is heat spreading and the other is to provide the electrical circuit layout for the application. On top of this is layer 104 an insulator to prevent surface short circuits and corrosion.
Typically, to get the required electrical isolation using a polyamide material (layer 102) of kilovolts, a 75 micron thick sheet is needed. For this the thermal conductivity is only 2.2 W/(m·K). This is adequate for power electronics where thermal load densities are of the order of Watts/square cm and a significant improvement in performance above FR4 circuit boards. However if this type of IMS-PCB is used with the LED placed directly on the PCB then high junction temperatures will occur as the thermal load will not be able to spread adequately in layer 103.
The IMS-PCB is widely used in the LED packaging industry as it can be used to mount ceramic packages which perform the function of heat spreading and thus make the thermal load equivalent to that of power electronics. In addition to this advantage the IMS-PCB can be machined with holes to allow mechanical attachment to a heat sink.
Of course, all these layers of packaging create extra cost and extra interfaces that increase thermal resistance. The best LEDs packaged in ceramic modules on IMS-PCBs provide a thermal resistance of about 8 K/W from the junction to the base of the module. An LED packaged in this way is shown in FIG. 2.
The metal substrate 201 has the adhesive layer 202 attaching a polyamide electrical insulation layer 203. On top of this is the metal circuit tracking layer 204. This assembly 201, 202, 203 and 204 is the IMS-PCB 221. On top of this is soldered or bonded using layer 205 the electrically insulating but thermally conducting ceramic tile (214) with the LED (212) attached by a solder or adhesive layer 213. The ceramic tile, 214, can be any number of ceramics such as alumina or aluminium nitride. The top electrical connection from the LED (212) to the electrical circuit layer 208 on the top of the ceramic tile (214) is via a wire bond 211. The electrical circuit layer 208 is in electrical contact with the bottom electrical circuit layer 206 through the use of an electrical via 207. The bottom electrical contact of the LED 212 is in electrical contact with the top electrical contact 215 of the ceramic tile through the use of a solder joint 213. This is in turn in electrical contact with the bottom side (217) of the ceramic tile 214 through the use of additional electrical via or vias 216.
The use of the thermally conductive ceramic tile 214 ensures that the large bottom contact 220 acts as the thermal path to the IMS PCB 221 but there is no electrical connection as no electrical via's are used in this section of the ceramic title 214. Thus, the top electrical contact to the LED and the bottom electrical contact to the LED are separated through the used of the IMS PCB 221 and any heat sink attached to the bottom side of the IMS PCB 221 is electrically isolated. This is an important issue if typical metal, graphite or conductive plastic heat sinks are used to prevent the heat sink becoming electrically live. The LED (212) is encapsulated with a non conducting epoxy or silicone encapsulant 210, held in a cup or receptacle 209 to allow good light extraction. Lenses are often used in addition, although this is not depicted here. The use of a ceramic tile allows for smooth surface to attach the LED onto, with LED solder joints being as thin as 3 um (gold tin solders) the surface morphology of the ceramic tile should be similar.
By cutting into the metal core of the IMS PCB and soldering a ceramic tile in direct contact with the core of the board the thermal resistance can be reduced. The best LEDs packaged using Aluminium Nitride ceramic tiles soldered into the core of the IMS PCB offer thermal resistance of 4 K/W from the junction to the base of the modules.
An LED packaged in this way is shown in FIG. 3. The metal substrate 301 has the adhesive layer 302 attaching a polyamide electrical insulation layer 303. On top of this is the metal circuit tracking layer 304. This assembly 301, 302, 303 and 304 is the IMS-PCB 316. The electrical tracking of the IMS-PCB 316 and the electrical circuit layer 306 of the ceramic tile 317 are electrically connected to together through the use of wire bonds 305. The ceramic tile, 317, can be any number of ceramics, such as alumina or aluminium nitride, although aluminium nitride is preferred due to its high thermal conductivity. The top electrical connection from the LED (309) to the electrical circuit layer 306 on the top of the ceramic tile (317) is via a wire bond 307. The bottom electrical contact of the LED 309 is in electrical contact with the top electrical contact 312 of the ceramic tile through the use of a solder joint 308.
The use of the thermally conductive ceramic tile 317 ensures that there is a low resistance thermal path to the metal substrate 301. Thus the top electrical contact to the LED and the bottom electrical contact to the LED are separated through the used of the IMS PCB 316 and any heat sink attached to the bottom side of the IMS PCB 316 is electrically isolated through the use of the ceramic tile 317. This is an important issue if typical metal, graphite or conductive plastic heat sinks are used to prevent the heat sink becoming electrically live. The LED (309) is encapsulated with a suitable encapsulant such non conducting epoxy or silicone encapsulant 310, held in a cup or receptacle 311 to allow good light extraction. Lenses are often used in addition, although this is not depicted here.
Despite developments in the field, there is a need for a simplified packaging of LEDs and a reduction in the thermal resistance path in the packaging.